1. Field of the Invention
The present invention relates generally to apparatus for chain-stitching materials and, particularly, to a stitching foot for chain-stitching composite structural materials with a single thread.
2. Description of the Prior Art
Sewing machines which, among other things, seam two or more layers of material to each other are notoriously old. Typically, such machines employ two threads which are interlocked in stitches formed by the sewing machine. In a typical prior art sewing machine of this type, shuttles or rotary hooks are employed on one side of the material with a needle alternately piercing and withdrawing from the material from its other side. In such machines, one thread is "threaded" through an eye in the needle while the other thread is contained in the shuttle or in a bobin associated with the rotary hook.
A difficulty encountered in sewing machines of the type described resides in establishing and maintaining the timing and position of the various components. For example, a reciprocating motion for the needle is established on one side of the material. A mechanical movement on the other side of the material must be timed and positioned with sufficient precision such that a shuttle can pass through a loop formed by the needle, or that loop can be engaged by the rotating hook. Generally, the mechanisms on both sides of the material are driven by a common power supply, such as an electric motor.
The noted timing and position problems increase with the span of the material to be stitched (without folding or otherwise doubling the material over on itself). This span is limited by the length of the linkages extending to the precision mechanisms on opposite sides of the material. Clearly, a greater span of material may be accommodated by extending the linkages. This, however, has practical limits given the precision necessary in the motion of the mechanisms and the fact that this precision is required on both sides of the material.
With many modern materials, discussed more fully below, it may not be possible (or desirable) to fold them for stitching. Thus, the span of materials that can be stitched is seriously limited by the required precision of the distinct stitch forming mechanisms. This is particularly true when those mechanisms are provided with the necessary strength (and attending mass) required to work with modern structural materials.
As an introduction, the above discussion is focused on a typical "two-thread" sewing machine. It is known, however, that a chain stitch sewing machine produces seams having greater resilience than the described two-thread devices. Thus, for structural applications, chain-stitching machines are very desirable.
Typical prior art chain stitch sewing machines work with a single thread that is "threaded" through the eye of a needle, the needle alternately piercing and withdrawing from the material being stitched from one side. A gripper hook operates on the other side of the material to grab a loop formed by the needle, as it pierces and withdraws from the material, and holds or retains that loop for passage or entry by the needle during the next piercing movement of the needle. Thus, while the described single-thread, chain stitch sewing machine produces seams having greater resilience, and therefore of greater value in the seaming of structural materials, such machines nonetheless require precision mechanisms on both sides of the material with the movement of those mechanisms being precisely timed. Thus, the limitations inherent in the prior art two thread machines described above, with regard to the span of material that may be accommodated, exist also in the described prior art chain-stitching machine.
Modern technology is developing many structural materials which it may be necessary, or desirable, to stitch for reinforcement or for the joining of structural components. For example, modern aircraft are made increasingly of non-metallic composite materials which have the requisite strength, without the weight of metal. Other advantages, such as the ability to avoid detection by radar, may also be possible with a reduction or elimination of metallic components. As indicated, composite structures of these types are known and in use. These structures may be laminated or otherwise formed so as to benefit from reinforcement by stitching at intervals along their span. In addition, structural components may be joined by stitching. In many cases, these materials are rigid and of significant dimension such that a sewing machine in accordance with the prior art, as described, requires significantly long dual linkages.